The pathogenic eukaryote Toxoplasma gondii causes particularly severe opportunistic infections in the context of immune suppression. These infections arise by the reactivation of chronic/latent tissue cysts, typically residing in the brai or CNS. Recrudescent infections are frequently fatal because the available drug treatments for acute T. gondii infection are suboptimal. Epidemiological studies have also linked chronic/latent T. gondii infections with neurological conditions such as schizophrenia in otherwise healthy immune competent adults. Thus, eradicating chronic/latent T. gondii infection provides an innovative approach to prevent recrudescent infections caused by immune suppression, as well as to reduce the potential of disease burden in immune competent individuals. Unfortunately, currently available treatments that target acute infection have little impact on the parasite tissu cyst stages that define chronic/latent infection. The identification of new drug targets that enabl innovative therapies to eliminate chronic/latent tissue cysts would have a major impact in the field. A major gap exists in our knowledge of parasite biology that is responsible for establishing and maintaining tissue cysts. There is also a major gap in understanding of the biological features of tissues cysts that are required for oral infectivity. GRA proteins are prominent proteins associated with the parasitophorous vacuole and the tissue cyst. GRA proteins are found uniquely in cyst-forming apicomplexans (Toxoplasma, Neospora, Sarcocystis) and represent potential novel drug targets that can be selectively targeted. We hypothesize GRA proteins that associate with the intravacuolar membranous network (IVN) of the parasitophorous vacuole provide critical biological functions required for cyst development and oral infectivity. I support of this hypothesis, we recently reported that two major dense granule (GRA) proteins of the IVN, GRA4 and GRA6, were required for development of normal cyst burdens in vivo. A role for IVN associated proteins in oral infectivity and transmission is hypothesized. Specific aim 1 will examine the hypothesis that IVN associated GRA proteins (GRA2, GRA3, GRA4, GRA6, GRA7, and GRA9) are required for differentiation, cyst development, and/or cyst maintenance in vivo. The role(s) of the IVN organizing amphipathic alpha helices of the GRA2 protein will be established by examining mutant versions of GRA2. Specific aim 2 will examine the hypotheses that IVN associated GRA proteins are required for oral infectivity and transmission. Collectively, these exploratory high impact aims will functionally identify essential roles that IVN associated GRA proteins play in the development of tissue cysts and in oral transmission. These aims will identify and validate new drug targets for innovative interventions to eradicate chronic/latent infection in already infected individuals. Consequently, this exploratory R21 project will have a lasting impact in the field by providing significant genetic tools and reagents, as well as by providing high impact biological information regarding drug target discovery that will guide the direction of future studies.